In reference to an article written by Master Howard Choy, a Zhongxin requires a boundary to locate, so the space in question has to be enclosed somehow to have a physical centre that we can locate with geometry, it is “below form” 形之下 that is a concrete thing. A Taiji is where the Yin and Yang meet, it could be the meeting place between the active and the passive and the substantial and the insubstantial of a house. In contrast, a Tianxin or a Heaven’s Heart is “above form” (an abstract idea); it can be looked upon like seeing the situation from heaven above to locate the reference point on earth below.
For the full article click: Where is your Loushu Centroid?
Source: The Feng Shui Architects Blog by Howard Choy